HV Power Systems, why they are better

In
basic terms, helicopters convert energy into mechanical power in order
to maintain flight. To expand, a specific helicopter flying a specific
way will require a specific amount of energy to perform the task. This
energy can be quantified as wattage on an electric model, and is
basically a constant we can use to compare different power systems in
order to show the benefits and drawbacks of each.

The power
system I am referring to is comprised of the motor, esc and battery. In
order to change from say a 3s to a 6s power system on a 450 sized heli,
one would have to replace all three components. A higher voltage esc is
needed, a properly sized lower Kv motor is needed and of course a
different sized battery pack is needed. Sometimes certain components
can be reused if they match up properly.

Starting with some known numbersLets
start with a common application where the power systems can range
anywhere from 6s to 12s, the 600 class helis. The most popular model
currently available is only available in a 6s power system from the
factory and is designed specifically to use a 6s 5000mAh flight pack.
Assuming a moderate head speed, we'll assume a 5min flight time using
80% of the pack. Now lets calculate the average wattage over this 5min
flight. Note: The following is covered in other sections on this site.

Now
that we have the average wattage over a 5min flight, we can use it to
compare different power systems that will work on this heli.

Increasing voltage by adding cellsWhat
does adding cells do? Well the more cells in series, the higher the
voltage but how does that help? Since wattage is basically a constant,
manipulating the voltage directly effects the current needed to achieve
that wattage.

Take the above example. The heli averaged 1065W
or 48A on a 6s pack. Now lets change the pack voltage and see how that
effects the amperage.

Watts = Volts * Ampsor we can rewrite it asAmps = Watts / Voltsand in this case everything is an average over a 5min flight so Avg. Amps = Avg. Watts / Avg. Volts

As
you can see the higher the voltage, the lower the amp draw. So what
happens if we go to even higher voltages? You guessed it, the amp draw
will yet again drop. Lastly lets look at the highest voltage system
that is commonly available for helis, 12s.

Avg. Amps = 1065W / 44.4V = 24A

On
first thought, it seem that the higher the voltage the better but there
is a practical limit. Most of this comes from the fact that the only
way to increase the voltage is to increase the number of cells and that
has limits.

Also as the number of cells raises, the capacity
of those cells needs to drop. The total amount of energy needs to
remain similar so the weight and size remain similar. For example a if
you are changing over the above example heli from a 6s power system to
a 12s power, the pack will need to change from a 6s 5000mAh pack to a
12s 2500mAh pack. Both packs offer the exact same energy (or Wh = Ah*V)
and should weight about the same.

Benefits of high voltage systemsThere
are many benefits to using a high voltage system (8s+) in helis and
there are also some drawbacks. Lets start with the benefits.

Lower amp drawOne
of the biggest benefits is the lower amperage needed. As we covered
above a 600 class heli with a 12s system only draws half the amperage
of a 6s system. This means the esc, wires and connectors can be much
smaller.

Higher voltage under loadThis
is where hardcore pilots jump on board. Because the amp draw is lower,
there is less of a voltage drop under load and that means higher
sustained voltage during hard flying. Simply put it makes it harder to
bog the heli.

Larger motors available for high voltage systemsThis
really has nothing to with the higher voltages but rather is a
byproduct of higher voltages being used in larger helis. You may have
noticed that the bigger/more powerful a motor is, the lower the kv (rpm
per volt) rating. Because each heli has a basically fixed, or at least
very limited adjustable, motor to main rotor ratio, a motor with the
proper kv is needed. By raising the voltage of the power system, a
lower kv motor is needed and this opens up more options.

For
example take a 600 class heli. One of the most popular models ships
with a 1220kv, 1600W motor designed for 6s. If you wanted to directly
replace it with a higher performance motor, the largest you could
choose is likely a 1100kv, 2200W motor from a popular motor company.
But if you were to move to a 12s power system, a motor with roughly
half the kv of the stock motor would be needed and that would open you
up to lots of new options including the largest heli motor this company
makes, a 630kv model rated at 4200W.

Drawbacks of high voltage systemsAlthough the drawbacks are small and few in comparison to the benefits, they do exist.

CostThis
biggest drawback is the cost. If you are starting from scratch, the
cost is not much more but if you already have a functioning power
system, the whole system will have to be replaced. These days the
battery cost is similar but the larger motor and HV esc will definitely
be more than the lower voltage models.

Powering the other componentsAnother
problem commonly faced how to power the receiver, servo and other
electronics. For 6s power systems, there are many BECs to choose from
but for 10s and 12s systems your choices become few. Another option is
to run a separate battery and regulator but that adds cost, weight and
an additional battery to charge. The good news is that HV BECs are
becoming more popular, so this is becoming less of an issue.

Bigger sparkMost
pilots are aware of the spark from connecting the lipo to the esc in a
6s power system and most of them don't like it. That said, the spark
from a 10s or 12s system is many times greater. This scares most pilots
but it is normal and should not be a concern.

Charging high voltage packs
Of course if you have a high voltage heli, you are going to have to
charge the high voltage packs. Chargers for high voltage packs can be
expensive and most are barely powerful enough to charge large packs at
1C. So this must be taken into account.

This
leads me into related subject, using 2 lower cell count packs in series
instead of 1 high cell count pack in high voltage setups.Using 2 packs for HV setupsThe
idea of using 2 smaller packs in series is becoming more and more
popular. Some believe that a single factory assembled pack is the only
option but this is simply not the truth. As such I believe the days of
huge packs, like a 12s 5000mAh pack, is soon coming to an end. Instead
it is better to use a pair of 6s 5000mAh packs in series.

Lower costThis
is a multi-part thing. The cost of the packs may or may not be cheaper,
that will depend on the packs chosen. Running 6s and smaller packs
opens up the option to use cheaper brand packs. Most of the 8s+ packs
are only available from a very limited number of assembles, most of
which only offer high end packs. That causes these packs to be very
expensive. So using 6s and smaller packs opens up many more brands
including many very cheap ones.

Crash worthinessWhen
you plant a 10s 600 sized heli from 100ft up, you can pretty guarantee
you taco'd the pack. If this was a 10s pack then the whole pack is
unusable but if it was a pair of 5s packs, there is a good chance one
of the 5s packs will survive. That saves money.

Pack placementHaving
a pair of flight packs allows you move them around. In some cases you
can separate them by placing one in the front of the heli and one in
the rear. This greatly aids the cg adjustment, as well as give you more
placement options for things like scale helis.

Charger choicesThis
is one of the biggest benefits, not having to buy a 10s or 12s charger
and instead allowing you to use a smaller charger or a pair of them for
faster charging. I am not going to explain all the shortcomings of the
current 12s chargers out there but just let me say that they are
expensive and limited. There are many more options for 10s chargers but
compared to the 6s charger market, they come up pretty short. Right now
the 6s chargers rule the roost. Using a pair of smaller packs lets you
choose from these chargers.

Much lower charge timeThis
is related to the charger choices. Because of the selection of 6s
chargers out there, it is easily possible to buy a pair of chargers and
use one for each pack. This allows much higher charge rates and
much lower charge times.

A few potential drawbacksBecause
you are using a pair of packs, care needs to be taken to make sure the
packs are well matched to each other. If you are buying new packs then
this is not a problem but if you have older packs or mis-matched packs,
it can affect performance. Next up is the more complicated wiring. A
pair of packs will have to wired to the esc with a y-adapter cable.
This slightly complicates things and adds additional points of failure.
These are both small issues that are easily outweighed by the benefits.

Sample wiring setups for dual packs

Running the bec off one pack.

In conclusionThe
future will be full of big electric helis and most will run on HV power
systems. Why? Because it makes no sense to use lower voltage systems
and the efficiency of a HV system offers so many benefits.